The respiratory gas that is delivered to persons who are ill usually is a mixture of air and oxygen and, in most cases, it is desirable to provide the gas to the patient at a temperature nearly equal to body temperature. Because body temperature almost always exceeds the temperature of the ambient atmosphere, the respiratory gas is heated.
It is required that respiratory gas be humidified. A positive means for adding humidity is required because both the air and oxygen are usually supplied from very dry sources of compressed air and compressed oxygen. Further, it is preferred that the humidity level be near 100 percent in most cases. That preference, combined with the requirement for a positive means for accomplishing humidification, has led to the near universal practice of humidifying respiratory gas by bubbling it through or passing it over heated water. Sometimes the gas is heated and it vaporizes the water. Whichever scheme is employed, the humidification is accomplished by bubbling gas through or passing it over water.
That scheme is simple and reasonably effective but it creates very serious practical problems. Unless the bubble path is very long, adequate humidification requires high temperature. If something goes amiss, the patient may receive very hot respiratory gas. In the case of an infant in an incubator or other enclosed environment, that is not easily detected and could be disastrous. Further, the pressure versus time plot of respiratory gas, as delivered to the infant, can very with water level in the humidifier. Low water level often results in higher gas temperature to the infant in some present designs. In addition, that old method presented a continuing nuisance problem to the nurses and others who attend infants being supplied with respiratory gas. Since the gas is supplied continuously at far higher volume than needed by the infant, and since it is humid and warmer than ambient atmosphere, condensation in large amounts occurs within the flow line. If the lines are arranged so that it is "all downhill" from humidifier to patient, the patient receives much condensate along with the respiratory gas. If it is not "all downhill," the condensate collects at the low point in the tubing. If the line is not drained periodically, condensate can disrupt the desired pressure of respiratory gas to the patient, and eventually be blown en masse up or down to the patient.